In semiconductor integrated circuit fabrication, deposition reactors employ a gas to deposit a film on a substrate. In chemical vapor deposition (CVD) process, a solid film is formed on a surface of the substrate. This film deposition occurs by a thermally activated reaction of gaseous species which are absorbed onto the surface of the substrate. However, the relatively high temperatures employed in CVD limit its applicability to certain process steps in the fabrication of semiconductor integrated circuits.
To avoid the relatively high temperatures of CVD, plasma-enhanced CVD (PECVD) was developed. Advantageously, with PECVD relatively low pressure and low temperature may be employed with relatively high electron energies with respect to heavy particle energies. This may be better understood by an understanding of a plasma.
A plasma is a collection of electrically charged particles and neutral particles. The charged particles of plasma have equal densities. In other words, the density of negatively charged particles (electrons and negative ions) is equal to the density of positively charged particles (positive ions). Plasma also comprises neutral particles or radicals. The radical is an atom or a molecule with unsatisfied chemical bonding having an equal number of electrons and protons. In a PECVD process, high electron temperatures are employed to increase the density of disassociated species within plasma. These disassociated species or radicals are available for deposition on a substrate assembly surface. Owing to a small mass, hot electrons do not create a high temperature process, as compared with a thermally activated CVD process. An enhanced supply of reactive free radicals makes deposition of high quality films possible at low temperatures as compared with a thermally activated CVD process. Moreover, high-density plasma may be employed to facilitate deposition. A high-density plasma is typically defined as having an ion-electron density on the order of 10.sup.10 -10.sup.13 ions-electrons per cm.sup.3 operating at 1 mtorr, where neutral to ion ratio is on the order of 100:1 to 1:1.
However, PECVD processes are somewhat problematic with respect to depositing conductive films. Such PECVD films tend to be relatively rough, and such films have a relatively high bulk resistance (&gt;250.mu..OMEGA.-cm). To overcome limitations in the character of PECVD deposited conductive films, a pulsed-plasma-enhanced CVD (PPECVD) process was developed. In such a process, a pulsed-plasma is provided by turning power "on" and "off". PPECVD allows for depositing reacting species onto a substrate and allows for departure of by-products from a substrate. Thus, a pulsed-plasma leads to a shift in dynamic equilibrium, which alters the average density of intermediate ion species present in the reaction chamber. These reaction kinetics have been employed for controlling depositing of conductive films with desired characteristics. However, control of such pulsed-plasmas is dependent on turning plasma power "on" and "off".
It has been suggested by others, that such pulsing is inefficient with respect to power consumption and transients. It has been suggested that by using interfering frequencies, a beat frequency may be introduced which facilitates amplitude modulation of a bias or driving signal for modulated plasma generation. Such beat frequency introduction has the advantage of potentially offering more control as compared to rapidly turning a plasma on and off, and may be used for PPECVD. It would be desirable to employ such a PPECVD approach in a manner, which provides greater control of deposition for selected deposition of species and separation of heavy ion species.